1. Field of the Invention
The present invention relates to a method of drying substrate, for example metal, such as steel, or plastic and wood, and the subsequent treatment of such dried substrate by various coating techniques. More particularly, an air filtration and drying system is used in the drying stage, comprising a booth in combination with means capable of generating an air stream which is sufficient to expeditiously reduce the moisture level on the substrate surface positioned in the booth. The resulting dried substrate has been found to have improved coating characteristics.
2. Description of Related Art
An air atomizing spray gun is typically utilized to rapidly apply paints, industrial coatings and other finishing products to a wide variety of industrial, commercial and consumer goods. Unfortunately, a profusion of transient, airborne particles and associated fumes, generally designated as overspray, are produced during the application process. To reduce the potentially serious health risks associated with the inhalation and bodily contact of the overspray, spray booths and other collection systems have been designed in accordance with a plethora of strict regulations. These regulations are set forth by the Occupational Safety and Health Administration (OSHA), the Environmental Protection Agency (EPA), the National Fire Protection Association (NFPA) and myriad other governmental regulatory agencies, to collect and effectively treat the discharged air and direct it away from the operators of the spray equipment and other adjacent ancillary personnel. Heretofore, high volume blowers have typically been utilized to draw uncontaminated, ambient air through the coating area, where the air mixes with the overspray, and to duct the air, now contaminated with coating particles and noxious gases, into a treatment area prior to discharge.
A dry filtration system, utilizing arrestor pads, has commonly been employed to remove overspray from the contaminated air stream. As the contaminated air stream passes through an arrestor pad, the larger coating particles impact against the surface of the pad and adhere thereto. As known in the art, the surfaces of the arrestor pad may be covered with an adhesive to facilitate the capture of the coating particles, thereby increasing the capture efficiency of the pad. The proper performance of arrestor pads in removing particles from a contaminated airstream is heavily dependent on frequent operator inspection and regularly performed maintenance. If the required inspections and maintenance are not performed according to specifications, arrestor pad blow by and an unintentional discharge of contaminants to the surrounding environment may occur.
A water-based overspray collection system, commonly designated as a water downfall system, utilizes a cascading curtain of water to remove overspray particles from a collection wall. The contaminated water is temporarily stored in a sump or collection tank and is subsequently pumped through a filter to remove any particles suspended therein. The filtered water may be reused in the water downfall system or may be discharged to a water treatment system or the environment. Prior to any discharge, the water must normally must go through an expensive and time consuming neutralization process, wherein any remaining particles in the water are allowed to sink to the bottom of the collection tank, thereby forming a concentrated sludge or cake that must be removed and disposed of on a regular basis.
The above-described overspray collection systems are moderately effective in the removal of larger overspray particles from the spray booth collection area. Unfortunately, they are not effective in the collection of submicron size particles and gases which are eventually discharged to the outside air, potentially creating an environmental hazard.
Solvent based coatings have commonly been utilized in finishing processes due to the fast drying characteristics of the solvents. As the solvents evaporate, the coating solids suspended therein flow together and form a continuous layer of dry solids. A major disadvantage of solvent based coatings is the explosion hazard created by the inherent flammability of the solvent and the associated solvent fumes which are released during the evaporation process. Additionally, the solvent fumes discharged to the atmosphere pose an environmental hazard due to the interaction of solvents with the ozone layer. As such, alternative coating processes utilizing dry powders, high solids and waterborne solids have been developed to avoid the disadvantages associated with solvent based coatings.
In particular coating applications, such as a dry powder coating process, an electrostatic spray gun assembly having a positive polarity is utilized to apply dry powder solids to a product having a negative polarity. Due to the resultant mutual attraction of the positively charged coating particles and the negatively charged substrate, overspray is substantially reduced. After receiving the dry paint particles, the coated product is baked at a high temperature until the dry paint particles melt and flow about the product, thereby forming a continuous coating. Such systems require substantial investment for equipment and have limited use due in part to the required baking step. In many instances, it is often required to wash the product prior to powder coating. As a result, the wet product has to be dried prior to coating to minimize any surface interaction with the applied coating particulates. An extended period of time is often required to dry the product to provide a surface suitable for coating with a powder or paint. Among the products which can be powder coated, there are included metal, for example, steel, plastic, such as a thermoplastic, or thermoset, and wood.
High solids coating systems utilize a high viscosity paint emulsion having a high solids to solvent ratio. As a result, the paint emulsion is generally applied to a product with a high pressure spray nozzle which inherently produces a substantial amount of overspray. The coated product is subsequently cured in a separate drying area using a heat source such as an oven or heat lamps. As with the above-described powder coating systems, a high solids coating system requires a substantial investment for equipment and has limited use due to the required heating step.
In a waterborne solids wet system, the coating solids are suspended in a fluid having a relatively high water to solvent ratio. Although the equipment required for this type of coating system is generally less expensive and complex due to a lower curing temperature, the required drying times are generally much longer than with solvent or dry powder based coatings.
As stated, currently available collection systems are generally designed to discharge large quantities of air to the outside environment. Unfortunately, this results in higher energy costs since additional energy must be expended to recondition the indoor building air. In addition, the residual pollutants in the discharged air are closely regulated by local and federal agencies, oftentimes requiring the procurement of a plurality of costly permits and/or the payment of large fines. These energy and regulatory requirements oftentimes add considerable cost to the price of a finished product.
Over the last decade, the use of high solvent based coatings has drastically decreased due to the ever increasing number of regulatory restrictions on the emission levels of contaminated air into the environment. As such, the popularity of dry powder, high solids, waterborne and other alternative coatings has increased tremendously. Due to the high investment cost and limitations of the dry powder and high solids coatings, waterborne coatings stand out as the best alternative for economical use. As stated above, one of the major disadvantages of a waterborne coating system is the requisite longer drying cycle which results in substantially increased production costs.
As described above, various coating techniques are available for coating a wide variety of substrates in an economical and environmentally safe manner. As previously discussed, certain coating methods moreover, such as powder coating, require a clean dry substrate prior to the application of the powder coating composition. However, available methods for drying wet substrate after it has been washed to prepare it for powder or liquid coating often require times such as 2 hours.
It would be desirable therefor to provide improved procedures for effectively drying substrate such as metal, for example steel, plastic, or wood to facilitate the subsequent application and cure of protective materials, such as used in powder coating, or water based acrylates.